Alginate bioink properties influence real-time impedance monitoring of cells during extrusion bioprinting

Bioprinting processes have greatly advanced in recent years through improvements in print accuracy and bioink optimization. Despite this progress, optimizing cell bioactivity still relies on guess-and-check processes with destructive testing post-printing. Measuring cell bioactivity during printing would improve print quality and inform complex printing processes, such as cell gradients or bioink transitions. Real-time monitoring using dielectric impedance spectroscopy alleviates this burden by correlating impedance |Z| to cell properties. However, the influence of bioink properties on these measurements is unknown. Using an in-line impedance sensor, we assessed the effect of alginate bioink concentration, pH, and crosslinking on impedance from 1 -25,000 kHz and determined how these properties influenced the detection of primary chondrocytes. Increasing the alginate concentration, decreasing the pH, or crosslinking with CaCl2 resulted in an increase in impedance. In nearly all samples, the addition of cells resulted in an increase in impedance compared to acellular samples, and this difference in impedance was used to quantify cell presence, termed |Zcells|. Higher alginate concentrations at 1 w/v% and 3 w/v% showed greater |Zcells|, indicating reliable cell detection. Although |Zcells| varied greatly with alginate or PBS pH, similar measurements were found in pH resembling cell media.Optimal frequency ranges for monitoring acellular and cellular samples were from 10 -100 kHz and 1,000 -25,000 kHz. Furthermore, cells were detected in real-time as acellular and cellular alginate bioinks were transitioned during bioprinting. This transition in cell concentration was spatially mapped to deposited bioink, providing a visual display of bioink transition using impedance. In summary, DIS was capable of detecting cells suspended in alginate bioink and showed potential for real-time mapping of cell deposition.